This invention relates to a light beam scanning apparatus to be suitably used for an image forming apparatus such as a digital copying machine or a laser printer adapted to scan a photosensitive drum by means of a laser beam for exposure and form an electrostatic latent image on the photosensitive drum.
In recent years, there have been developed various digital copying machines provided with a light beam scanning apparatus for scanning an object by means of a laser beam (to be referred to simply as light beam) for exposure adapted to form an image by way of an electrophotography process.
There have been also developed various digital copying machines provided with a multi-beam type light beam scanning apparatus adapted to generate a plurality of light beams and scanning an object for a plurality of lines simultaneously by means of the plurality of light beams in order to achieve a high image forming rate.
Conventionally, a digital copying machine provided with such a beam scanning apparatus or some other comparable optical unit is controlled for the position of exposure to a light beam in the scanning direction (light beam position control in the main scanning direction) and also for the passing position of the light beam (light beam position control in the sub scanning direction) for the purpose of forming a high quality image.
Japanese Patent Applications Nos. 09-667317 and 09-816773 propose specific position control techniques to be used for a light beam in an optical unit of the type under consideration. They utilize a pair of optical sensors arranged in juxtaposition in the scanning direction of the light beam with a predetermined gap separating them in order to detect the passing position of the light beam. Of the above identified patent documents, Japanese Patent Application No. 09-667317 describes the use of a pair of sensors provided respectively with trapezoidal or triangular patterns arranged symmetrically relative to each other. On the other hand, Japanese Patent Application No. 09-816773 describes the use of a pair of sensors provided respectively with saw-toothed patterns formed by arranging trapezoids or triangles and arranged interdigitally and symmetrically relative to each other.
According to either of the above patent documents, the paired sensors are adapted to convert the detected light beam into respective electric signals. Then, in the light beam scanning apparatus, the difference between the electric signals from the respective sensors is integrated by an integrator to detect the scanning position of the light beam. For improving the detection accuracy of such a conventional light beam scanning apparatus, the sensitivity of the sensors and the integrator needs to be improved and the amplification factor of the signal processing circuit has to be raised.
However, any effort for improving the detection accuracy of a conventional light beam scanning apparatus ends up in the fact that the output signal of the integrator becomes disproportionately large if the light beam is displaced only slightly from an intended position. Then, the output signal of the integrator can be mostly held to or near the supply voltage level or the ground level. In other words, any effort for improving the detection accuracy of a conventional light beam scanning apparatus is accompanied by the problem of very narrow detection range and/or the difficulty of adapting itself to changes with time including environmental changes. Therefore, there is a strong demand for light beam scanning apparatus that can easily adapt themselves to changes with time and show a high detection accuracy and a wide detection range.
In view of the above identified circumstances, it is therefore the object of the present invention to provide a light beam scanning apparatus that shows a high detection accuracy and a broad detection range.
According to the invention, the above object is achieved by providing a light beam scanning apparatus for scanning a surface to be scanned by means of a light beam, the apparatus comprising: a light emitting unit which outputs a light beam; a scanning member which causes the light beam output from the light emitting unit to scan the surface to be scanned in a main scanning direction; a light beam detecting section which has a pair of symmetrical saw-toothed patterns with a narrowed width for marginally located teeth and are formed by arranging optical elements for converting the light beam into respective electric signals, the patterns being arranged interdigitally on the surface to be scanned or at a position equivalent to the surface; a processing circuit which outputs the integral value obtained by integrating the difference of the electric signals output from the respective patterns of the light beam detecting section; and a CPU which determines the passing position of the light beam output from the light emitting unit passes on the surface to be scanned.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.